Laser desorption and ionization have been utilized to ablate and ionize a wide variety of surface samples for analysis with mass spectrometry. Matrix-assisted laser desorption/ionization (MALDI) is a desorption and ionization technique that results in production of gas-phase ions from condensed-phase analyte molecules (e.g. generally large labilte biomolecules) by unique energy partitioning properties of absorbed light from lasers into target sample components. MALDI samples are generally mixtures of matrix and analyte, whereby the light energy from the laser is absorbed primarily by the matrix, facilitating both ionization and desorption of analyte. The beneficial characteristic of these processes is that very little of the energy is partitioned into the internal energy of the analyte, resulting in intact gas-phase analyte ions. Gas-phase anayte ions are generally analyzed by time-of-flight mass spectrometers; however, any number of gas-phase ion analyzers have been considered and employed for MALDI analysis.
The technique of MALDI developed primarily from research by Karas and Hillenkamp (1) in the late 1980. Vacuum MALDI has developed into a widely used commercial technology for analysis of proteins and other macromolecules.
The present invention relates to the application of MALDI to desorption and ionization in vacuum and at intermediate and higher pressures, including atmospheric pressure. Franzen and Koster (U.S. Pat. No. 5,663,561) first described atmospheric pressure MALDI in reference to their atmospheric pressure desorption/ionization technique by stating, “In contrast to MALDI, at atmospheric pressure, the related molecules of the decomposed matrix material are not needed to ionize the macromolecules. The selection of matrix molecules is solely dependent upon their ability to release the large molecules.” Albeit, not explicitly claimed in this patent, the concept of atmospheric pressure MALDI (or AP-MALDI) was clearly first described by Franzen and Koster. Ironically, the Franzen and Koster patent begins by arguing that AP-MALDI is inefficient and that augmenting ionization efficiency with gas phase ion-molecule reactions or desorbed neutral species with gas phase reagent ions at atmospheric pressure would offset some of the transmission losses that would occur by inefficient transport from atmospheric pressure.
Laiko and Burlingame (U.S. Pat. No. 5,965,884) distinguish their AP-MALDI from Franzen and Koster by arguing simplicity and non-destructive matrices. This patent dismisses the key arguments made by Franzen and Koster that AP-MALDI is inefficient. The Laiko patent teaches AP-MALDI with the requirement of close coupling of a sample target to the conductance aperture into vacuum. The lack of efficient atmospheric pressure optics with this device requires precise alignment and positioning of sample and the laser beam relative to the vacuum inlet. In addition, Laiko provides for a sweep gas to assist in transport of the ions from the target surface to the vacuum inlet. The transmission of this device is low. The lack of time-sequenced optics with the laser pulse limit ion extraction and transmission efficiency.
Sheehan and Willoughby (U.S. Pat. No. 6,744,041 B2) describe separation of the ionization process [and sample target position] from the conductance aperture using atmospheric pressure optics. They describe efficient atmospheric pressure transport and compression optics that allow relative independence of sample location from the position of the vacuum inlet. Components of this invention are included by reference into the present invention.
Sheehan and Willoughby (U.S. RPA Ser. No. 10/449,147) describe further improvement of transmission of MALDI generated ions at atmospheric pressure by laminating high transmission elements and incorporating a “back-well” geometry whereby MALDI samples can be placed facing away from the conductance aperture. This geometry facilitates easier access of the laser beam to the sample targets compared to close-coupled designs. The back-well geometry also provides a simplification of sample insertion and easier access to the ionization chamber. Components of this invention are also included by reference into the present invention.
Willoughby and Sheehan (U.S. RPA 60/419,699) also describe improvements in transmission of ions from atmospheric pressure sources [including AP-MALDI]. These improvements are accomplished by precisely controlling the electric field through the entire conductance pathway from atmospheric pressure into vacuum. Components of this invention are included by reference into the present invention. Willoughby and Sheehan (U.S. PPA 60/476,582) also teach that conductance arrays and patterned optics can further enhance the transmission of ions from atmospheric pressure sources and improve the transmission of MALDI ions from either intermediate of higher-pressure sources. Components of this invention are included by reference into the present invention.
Whitehouse (US 20020175278) describes the use of a variety of RF multipole devices and DC funnel devices to focus and entrain the flow of ions from atmospheric and intermediate pressure MALDI targets to detection. Components of this invention are included by reference into the present invention.
Truche et al. (U.S. Pat. No. 6,707,039 B1) describe a wide variety of alternatives for close-coupling the sample target to the conductance aperture. This technology places high tolerance on sample position and laser position. In addition, it is envisioned that mirrored reflective surfaces close to the plume of the MALDI target would tend to become contaminated and degraded in their optical performance. In addition, the sampling of ions from an electric field between the target and aperture into the field-free region of the vacuum inlet tube would cause rim losses from field penetration and degrade the transport efficiency. The lack of time-sequenced optics with the laser pulse limit ion extraction and transmission efficiency.
Makarov and Bondarenko (U.S. Pat. No. 6,707,036 B2) teach of a positionally optimized sample target device with a close-coupled conductance opening for atmospheric pressure and intermediate pressure MALDI. This device is still subordinate to alignment of laser, target, and lacks spatial or temporal optics to facilitate efficient ion transmission to the mass analyzer. The lack of time-sequenced optics with the laser pulse limit ion extraction and transmission efficiency.                1. Karas, M.; Hillenkamp, F., Anal. Chem. 1988, 60, 2299-2301.        